1. Field of the Invention
The present invention relates to autoconverters utilized in power supply devices, and in particular to a circuit arrangement for connecting two autoconverters in a switched power supply.
2. Description of the Prior Art
Power supply devices having autoconverters are generally known. So-called switched power packs have become particularly widespread, wherein an input d.c. voltage (usually a rectified line voltage) is periodically connected with relatively high frequency to an inductance by means of a load switch, preferably in the form of a semiconductor element. The inductance in turn supplies a user with power during blocking phases of the load switch. Two basic types of autoconverters are utilized, namely blocking and flow autoconverters. These two types are essentially differentiated in that the inductance is disposed in a shunt branch of a blocking autoconverter, and in a series branch of a flow autoconverter. In the case of a blocking autoconverter, the load resistor is fed exclusively by the discharge current of the inductance, whereby the current pauses are bridged from a capacitor connected in parallel with the load resistor. In the case of a flow autoconverter, the load resistor is supplied with the charging current of the inductance during the conducting phase of the load switch, and continues to be fed by the discharge current of the inductance through a diode during the non-conducting or off phase of the load switch. Autoconverter circuits further generally include a transformer for galvanic separation of the input or output circuit. In a blocking autoconverter, this isolating transformer may simultaneously form the storage inductance.
In addition to depending upon the voltage ratio of an isolating transformer which may be provided, the output voltage of an autoconverter depends on the so-called keying ratio, that is, the pulse duty factor with which the load switch is switched on and off. The output voltage may, therefore, be controlled with respect to a predetermined value by modifying the keying ratio. It is known to operate the load switch at a constant frequency and to control the output voltage by means of pulse duration modulation of the switch pulses of the load switch. It is known from German OS No. 2520631 in this manner of operation to limit the region within which a pulse duration modulation is possible as a function of the input d.c. voltage, in order to prevent an overload of the respective components.
Blocking converters are generally utilized for comparatively low outputs, whereas flow autoconverters are suitable for building switched power supplies of moderate and higher outputs. High output switched power packs are known wherein two flow autoconverters are connected in parallel at their inputs and outputs. This arrangement, referred to as a "push-push converter" is essentially characterized by the two load switches being switched with a chronological offset, so that the ripple of the output current has twice the frequency as in a normal (single-ended) flow autoconverter.
The power which is transmittable by switched power packs constructed with autoconverters is limited by the load handling capability of the available load switches, which are generally formed by transistors. The limit established by the current handling capability of these switch elements may be overcome by connecting two or more autoconverters in parallel as described above. Another limit on the transmittable power is caused by the feed network. Conventional switched power packs are supplied from a rectified 220 volt single-phase alternating voltage. Commercially available switching transistors, usable as load switches, are suitable for this voltage and for current loads corresponding to a power pack output of up to 2 kV. Higher outputs cause undesirable influences on the feed network, for example, as a consequence of the unavoidably high input capacitance, a very large non-linear distortion factor of the current results, a very high cut-in current surge also occurs, and the neutral conductor is very highly loaded given a single-phase mains connection.
These disadvantages suggest feeding extremely high power output switched power packs with a three-phase network. Connection to, for example, a 380 volt network, however, requires a correspondingly higher rectified input voltage for the autoconverter. If a switched power pack to be supplied with a three-phase network were constructed using flow autoconverters, transistors having a maximum collector, emitter voltage of approximately 1600 volts would be necessary for use as load switches. Components having such current handling capability and switching speeds are not commercially available at present.